The Gemmorah (Succah 3 and Mishna on 28) states that the requirements for a Succah are that a person can fit it "ראשו ורובו" - the head and most of the body.

IIRC the average body is 3 Amot (18 Tfochim)Gemorrah elsewhere and the head is about half Ama (3 Tfochim) for a total of 21 Tfochim (5'9 or 175cm). So, Rosho (3 Tf.) + most of his body (10 Tf) would equal 13 Tfochim.

We know that they ate laying horizontally on beds, otherwise, if one sits or stands vertically, IMHO, Rosho AND Rubo is meaningless, as the head is above the body.

I always understood רובו to mean most of the torso.
– DonielFSep 12 '19 at 12:42

@DonielF Hm, great, I think that counts as an answer! Maybe somebody will find a source.
– Al BerkoSep 12 '19 at 12:49

"average body is 3 Amot" in which dimension? Most of us aren't spherical
– Double AA♦Sep 12 '19 at 12:54

The average body is 3x1x1, which is how we get that a miqvah needs to be 40 se'ah of water. 3 cubic amos. 10 x 7 x 7 techaim would be 2-29/108 cubic amos. Which is well more than rov. But then, it's not rov of the head, it's ALL of the head and rov of everything else. I don't think you need to exclude arms and legs for this to work; you just have to use volume, not linear measurements.
– Micha BergerSep 12 '19 at 17:06